1Shared Subtrees
2---------------
3 4Contents:
5 1) Overview
6 2) Features
7 3) Setting mount states
8 4) Use-case
9 5) Detailed semantics
10 6) Quiz
11 7) FAQ
12 8) Implementation
13 14 151) Overview
16-----------
17 18Consider the following situation:
19 20A process wants to clone its own namespace, but still wants to access the CD
21that got mounted recently. Shared subtree semantics provide the necessary
22mechanism to accomplish the above.
23 24It provides the necessary building blocks for features like per-user-namespace
25and versioned filesystem.
26 272) Features
28-----------
29 30Shared subtree provides four different flavors of mounts; struct vfsmount to be
31precise
32 33 a. shared mount
34 b. slave mount
35 c. private mount
36 d. unbindable mount
37 38 392a) A shared mount can be replicated to as many mountpoints and all the
40replicas continue to be exactly same.
41 42 Here is an example:
43 44 Let's say /mnt has a mount that is shared.
45 mount --make-shared /mnt
46 47 Note: mount(8) command now supports the --make-shared flag,
48 so the sample 'smount' program is no longer needed and has been
49 removed.
50 51 # mount --bind /mnt /tmp
52 The above command replicates the mount at /mnt to the mountpoint /tmp
53 and the contents of both the mounts remain identical.
54 55 #ls /mnt
56 a b c
57 58 #ls /tmp
59 a b c
60 61 Now let's say we mount a device at /tmp/a
62 # mount /dev/sd0 /tmp/a
63 64 #ls /tmp/a
65 t1 t2 t3
66 67 #ls /mnt/a
68 t1 t2 t3
69 70 Note that the mount has propagated to the mount at /mnt as well.
71 72 And the same is true even when /dev/sd0 is mounted on /mnt/a. The
73 contents will be visible under /tmp/a too.
74 75 762b) A slave mount is like a shared mount except that mount and umount events
77 only propagate towards it.
78 79 All slave mounts have a master mount which is a shared.
80 81 Here is an example:
82 83 Let's say /mnt has a mount which is shared.
84 # mount --make-shared /mnt
85 86 Let's bind mount /mnt to /tmp
87 # mount --bind /mnt /tmp
88 89 the new mount at /tmp becomes a shared mount and it is a replica of
90 the mount at /mnt.
91 92 Now let's make the mount at /tmp; a slave of /mnt
93 # mount --make-slave /tmp
94 95 let's mount /dev/sd0 on /mnt/a
96 # mount /dev/sd0 /mnt/a
97 98 #ls /mnt/a
99 t1 t2 t3
100 101 #ls /tmp/a
102 t1 t2 t3
103 104 Note the mount event has propagated to the mount at /tmp
105 106 However let's see what happens if we mount something on the mount at /tmp
107 108 # mount /dev/sd1 /tmp/b
109 110 #ls /tmp/b
111 s1 s2 s3
112 113 #ls /mnt/b
114 115 Note how the mount event has not propagated to the mount at
116 /mnt
117 118 1192c) A private mount does not forward or receive propagation.
120 121 This is the mount we are familiar with. Its the default type.
122 123 1242d) A unbindable mount is a unbindable private mount
125 126 let's say we have a mount at /mnt and we make is unbindable
127 128 # mount --make-unbindable /mnt
129 130 Let's try to bind mount this mount somewhere else.
131 # mount --bind /mnt /tmp
132 mount: wrong fs type, bad option, bad superblock on /mnt,
133 or too many mounted file systems
134 135 Binding a unbindable mount is a invalid operation.
136 137 1383) Setting mount states
139 140 The mount command (util-linux package) can be used to set mount
141 states:
142 143 mount --make-shared mountpoint
144 mount --make-slave mountpoint
145 mount --make-private mountpoint
146 mount --make-unbindable mountpoint
147 148 1494) Use cases
150------------
151 152 A) A process wants to clone its own namespace, but still wants to
153 access the CD that got mounted recently.
154 155 Solution:
156 157 The system administrator can make the mount at /cdrom shared
158 mount --bind /cdrom /cdrom
159 mount --make-shared /cdrom
160 161 Now any process that clones off a new namespace will have a
162 mount at /cdrom which is a replica of the same mount in the
163 parent namespace.
164 165 So when a CD is inserted and mounted at /cdrom that mount gets
166 propagated to the other mount at /cdrom in all the other clone
167 namespaces.
168 169 B) A process wants its mounts invisible to any other process, but
170 still be able to see the other system mounts.
171 172 Solution:
173 174 To begin with, the administrator can mark the entire mount tree
175 as shareable.
176 177 mount --make-rshared /
178 179 A new process can clone off a new namespace. And mark some part
180 of its namespace as slave
181 182 mount --make-rslave /myprivatetree
183 184 Hence forth any mounts within the /myprivatetree done by the
185 process will not show up in any other namespace. However mounts
186 done in the parent namespace under /myprivatetree still shows
187 up in the process's namespace.
188 189 190 Apart from the above semantics this feature provides the
191 building blocks to solve the following problems:
192 193 C) Per-user namespace
194 195 The above semantics allows a way to share mounts across
196 namespaces. But namespaces are associated with processes. If
197 namespaces are made first class objects with user API to
198 associate/disassociate a namespace with userid, then each user
199 could have his/her own namespace and tailor it to his/her
200 requirements. Offcourse its needs support from PAM.
201 202 D) Versioned files
203 204 If the entire mount tree is visible at multiple locations, then
205 a underlying versioning file system can return different
206 version of the file depending on the path used to access that
207 file.
208 209 An example is:
210 211 mount --make-shared /
212 mount --rbind / /view/v1
213 mount --rbind / /view/v2
214 mount --rbind / /view/v3
215 mount --rbind / /view/v4
216 217 and if /usr has a versioning filesystem mounted, then that
218 mount appears at /view/v1/usr, /view/v2/usr, /view/v3/usr and
219 /view/v4/usr too
220 221 A user can request v3 version of the file /usr/fs/namespace.c
222 by accessing /view/v3/usr/fs/namespace.c . The underlying
223 versioning filesystem can then decipher that v3 version of the
224 filesystem is being requested and return the corresponding
225 inode.
226 2275) Detailed semantics:
228-------------------
229 The section below explains the detailed semantics of
230 bind, rbind, move, mount, umount and clone-namespace operations.
231 232 Note: the word 'vfsmount' and the noun 'mount' have been used
233 to mean the same thing, throughout this document.
234 2355a) Mount states
236 237 A given mount can be in one of the following states
238 1) shared
239 2) slave
240 3) shared and slave
241 4) private
242 5) unbindable
243 244 A 'propagation event' is defined as event generated on a vfsmount
245 that leads to mount or unmount actions in other vfsmounts.
246 247 A 'peer group' is defined as a group of vfsmounts that propagate
248 events to each other.
249 250 (1) Shared mounts
251 252 A 'shared mount' is defined as a vfsmount that belongs to a
253 'peer group'.
254 255 For example:
256 mount --make-shared /mnt
257 mount --bind /mnt /tmp
258 259 The mount at /mnt and that at /tmp are both shared and belong
260 to the same peer group. Anything mounted or unmounted under
261 /mnt or /tmp reflect in all the other mounts of its peer
262 group.
263 264 265 (2) Slave mounts
266 267 A 'slave mount' is defined as a vfsmount that receives
268 propagation events and does not forward propagation events.
269 270 A slave mount as the name implies has a master mount from which
271 mount/unmount events are received. Events do not propagate from
272 the slave mount to the master. Only a shared mount can be made
273 a slave by executing the following command
274 275 mount --make-slave mount
276 277 A shared mount that is made as a slave is no more shared unless
278 modified to become shared.
279 280 (3) Shared and Slave
281 282 A vfsmount can be both shared as well as slave. This state
283 indicates that the mount is a slave of some vfsmount, and
284 has its own peer group too. This vfsmount receives propagation
285 events from its master vfsmount, and also forwards propagation
286 events to its 'peer group' and to its slave vfsmounts.
287 288 Strictly speaking, the vfsmount is shared having its own
289 peer group, and this peer-group is a slave of some other
290 peer group.
291 292 Only a slave vfsmount can be made as 'shared and slave' by
293 either executing the following command
294 mount --make-shared mount
295 or by moving the slave vfsmount under a shared vfsmount.
296 297 (4) Private mount
298 299 A 'private mount' is defined as vfsmount that does not
300 receive or forward any propagation events.
301 302 (5) Unbindable mount
303 304 A 'unbindable mount' is defined as vfsmount that does not
305 receive or forward any propagation events and cannot
306 be bind mounted.
307 308 309 State diagram:
310 The state diagram below explains the state transition of a mount,
311 in response to various commands.
312 ------------------------------------------------------------------------
313 | |make-shared | make-slave | make-private |make-unbindab|
314 --------------|------------|--------------|--------------|-------------|
315 |shared |shared |*slave/private| private | unbindable |
316 | | | | | |
317 |-------------|------------|--------------|--------------|-------------|
318 |slave |shared | **slave | private | unbindable |
319 | |and slave | | | |
320 |-------------|------------|--------------|--------------|-------------|
321 |shared |shared | slave | private | unbindable |
322 |and slave |and slave | | | |
323 |-------------|------------|--------------|--------------|-------------|
324 |private |shared | **private | private | unbindable |
325 |-------------|------------|--------------|--------------|-------------|
326 |unbindable |shared |**unbindable | private | unbindable |
327 ------------------------------------------------------------------------
328 329 * if the shared mount is the only mount in its peer group, making it
330 slave, makes it private automatically. Note that there is no master to
331 which it can be slaved to.
332 333 ** slaving a non-shared mount has no effect on the mount.
334 335 Apart from the commands listed below, the 'move' operation also changes
336 the state of a mount depending on type of the destination mount. Its
337 explained in section 5d.
338 3395b) Bind semantics
340 341 Consider the following command
342 343 mount --bind A/a B/b
344 345 where 'A' is the source mount, 'a' is the dentry in the mount 'A', 'B'
346 is the destination mount and 'b' is the dentry in the destination mount.
347 348 The outcome depends on the type of mount of 'A' and 'B'. The table
349 below contains quick reference.
350 ---------------------------------------------------------------------------
351 | BIND MOUNT OPERATION |
352 |**************************************************************************
353 |source(A)->| shared | private | slave | unbindable |
354 | dest(B) | | | | |
355 | | | | | | |
356 | v | | | | |
357 |**************************************************************************
358 | shared | shared | shared | shared & slave | invalid |
359 | | | | | |
360 |non-shared| shared | private | slave | invalid |
361 ***************************************************************************
362 363 Details:
364 365 1. 'A' is a shared mount and 'B' is a shared mount. A new mount 'C'
366 which is clone of 'A', is created. Its root dentry is 'a' . 'C' is
367 mounted on mount 'B' at dentry 'b'. Also new mount 'C1', 'C2', 'C3' ...
368 are created and mounted at the dentry 'b' on all mounts where 'B'
369 propagates to. A new propagation tree containing 'C1',..,'Cn' is
370 created. This propagation tree is identical to the propagation tree of
371 'B'. And finally the peer-group of 'C' is merged with the peer group
372 of 'A'.
373 374 2. 'A' is a private mount and 'B' is a shared mount. A new mount 'C'
375 which is clone of 'A', is created. Its root dentry is 'a'. 'C' is
376 mounted on mount 'B' at dentry 'b'. Also new mount 'C1', 'C2', 'C3' ...
377 are created and mounted at the dentry 'b' on all mounts where 'B'
378 propagates to. A new propagation tree is set containing all new mounts
379 'C', 'C1', .., 'Cn' with exactly the same configuration as the
380 propagation tree for 'B'.
381 382 3. 'A' is a slave mount of mount 'Z' and 'B' is a shared mount. A new
383 mount 'C' which is clone of 'A', is created. Its root dentry is 'a' .
384 'C' is mounted on mount 'B' at dentry 'b'. Also new mounts 'C1', 'C2',
385 'C3' ... are created and mounted at the dentry 'b' on all mounts where
386 'B' propagates to. A new propagation tree containing the new mounts
387 'C','C1',.. 'Cn' is created. This propagation tree is identical to the
388 propagation tree for 'B'. And finally the mount 'C' and its peer group
389 is made the slave of mount 'Z'. In other words, mount 'C' is in the
390 state 'slave and shared'.
391 392 4. 'A' is a unbindable mount and 'B' is a shared mount. This is a
393 invalid operation.
394 395 5. 'A' is a private mount and 'B' is a non-shared(private or slave or
396 unbindable) mount. A new mount 'C' which is clone of 'A', is created.
397 Its root dentry is 'a'. 'C' is mounted on mount 'B' at dentry 'b'.
398 399 6. 'A' is a shared mount and 'B' is a non-shared mount. A new mount 'C'
400 which is a clone of 'A' is created. Its root dentry is 'a'. 'C' is
401 mounted on mount 'B' at dentry 'b'. 'C' is made a member of the
402 peer-group of 'A'.
403 404 7. 'A' is a slave mount of mount 'Z' and 'B' is a non-shared mount. A
405 new mount 'C' which is a clone of 'A' is created. Its root dentry is
406 'a'. 'C' is mounted on mount 'B' at dentry 'b'. Also 'C' is set as a
407 slave mount of 'Z'. In other words 'A' and 'C' are both slave mounts of
408 'Z'. All mount/unmount events on 'Z' propagates to 'A' and 'C'. But
409 mount/unmount on 'A' do not propagate anywhere else. Similarly
410 mount/unmount on 'C' do not propagate anywhere else.
411 412 8. 'A' is a unbindable mount and 'B' is a non-shared mount. This is a
413 invalid operation. A unbindable mount cannot be bind mounted.
414 4155c) Rbind semantics
416 417 rbind is same as bind. Bind replicates the specified mount. Rbind
418 replicates all the mounts in the tree belonging to the specified mount.
419 Rbind mount is bind mount applied to all the mounts in the tree.
420 421 If the source tree that is rbind has some unbindable mounts,
422 then the subtree under the unbindable mount is pruned in the new
423 location.
424 425 eg: let's say we have the following mount tree.
426 427 A
428 / \
429 B C
430 / \ / \
431 D E F G
432 433 Let's say all the mount except the mount C in the tree are
434 of a type other than unbindable.
435 436 If this tree is rbound to say Z
437 438 We will have the following tree at the new location.
439 440 Z
441 |
442 A'
443 /
444 B' Note how the tree under C is pruned
445 / \ in the new location.
446 D' E'
447 448 449 4505d) Move semantics
451 452 Consider the following command
453 454 mount --move A B/b
455 456 where 'A' is the source mount, 'B' is the destination mount and 'b' is
457 the dentry in the destination mount.
458 459 The outcome depends on the type of the mount of 'A' and 'B'. The table
460 below is a quick reference.
461 ---------------------------------------------------------------------------
462 | MOVE MOUNT OPERATION |
463 |**************************************************************************
464 | source(A)->| shared | private | slave | unbindable |
465 | dest(B) | | | | |
466 | | | | | | |
467 | v | | | | |
468 |**************************************************************************
469 | shared | shared | shared |shared and slave| invalid |
470 | | | | | |
471 |non-shared| shared | private | slave | unbindable |
472 ***************************************************************************
473 NOTE: moving a mount residing under a shared mount is invalid.
474 475 Details follow:
476 477 1. 'A' is a shared mount and 'B' is a shared mount. The mount 'A' is
478 mounted on mount 'B' at dentry 'b'. Also new mounts 'A1', 'A2'...'An'
479 are created and mounted at dentry 'b' on all mounts that receive
480 propagation from mount 'B'. A new propagation tree is created in the
481 exact same configuration as that of 'B'. This new propagation tree
482 contains all the new mounts 'A1', 'A2'... 'An'. And this new
483 propagation tree is appended to the already existing propagation tree
484 of 'A'.
485 486 2. 'A' is a private mount and 'B' is a shared mount. The mount 'A' is
487 mounted on mount 'B' at dentry 'b'. Also new mount 'A1', 'A2'... 'An'
488 are created and mounted at dentry 'b' on all mounts that receive
489 propagation from mount 'B'. The mount 'A' becomes a shared mount and a
490 propagation tree is created which is identical to that of
491 'B'. This new propagation tree contains all the new mounts 'A1',
492 'A2'... 'An'.
493 494 3. 'A' is a slave mount of mount 'Z' and 'B' is a shared mount. The
495 mount 'A' is mounted on mount 'B' at dentry 'b'. Also new mounts 'A1',
496 'A2'... 'An' are created and mounted at dentry 'b' on all mounts that
497 receive propagation from mount 'B'. A new propagation tree is created
498 in the exact same configuration as that of 'B'. This new propagation
499 tree contains all the new mounts 'A1', 'A2'... 'An'. And this new
500 propagation tree is appended to the already existing propagation tree of
501 'A'. Mount 'A' continues to be the slave mount of 'Z' but it also
502 becomes 'shared'.
503 504 4. 'A' is a unbindable mount and 'B' is a shared mount. The operation
505 is invalid. Because mounting anything on the shared mount 'B' can
506 create new mounts that get mounted on the mounts that receive
507 propagation from 'B'. And since the mount 'A' is unbindable, cloning
508 it to mount at other mountpoints is not possible.
509 510 5. 'A' is a private mount and 'B' is a non-shared(private or slave or
511 unbindable) mount. The mount 'A' is mounted on mount 'B' at dentry 'b'.
512 513 6. 'A' is a shared mount and 'B' is a non-shared mount. The mount 'A'
514 is mounted on mount 'B' at dentry 'b'. Mount 'A' continues to be a
515 shared mount.
516 517 7. 'A' is a slave mount of mount 'Z' and 'B' is a non-shared mount.
518 The mount 'A' is mounted on mount 'B' at dentry 'b'. Mount 'A'
519 continues to be a slave mount of mount 'Z'.
520 521 8. 'A' is a unbindable mount and 'B' is a non-shared mount. The mount
522 'A' is mounted on mount 'B' at dentry 'b'. Mount 'A' continues to be a
523 unbindable mount.
524 5255e) Mount semantics
526 527 Consider the following command
528 529 mount device B/b
530 531 'B' is the destination mount and 'b' is the dentry in the destination
532 mount.
533 534 The above operation is the same as bind operation with the exception
535 that the source mount is always a private mount.
536 537 5385f) Unmount semantics
539 540 Consider the following command
541 542 umount A
543 544 where 'A' is a mount mounted on mount 'B' at dentry 'b'.
545 546 If mount 'B' is shared, then all most-recently-mounted mounts at dentry
547 'b' on mounts that receive propagation from mount 'B' and does not have
548 sub-mounts within them are unmounted.
549 550 Example: Let's say 'B1', 'B2', 'B3' are shared mounts that propagate to
551 each other.
552 553 let's say 'A1', 'A2', 'A3' are first mounted at dentry 'b' on mount
554 'B1', 'B2' and 'B3' respectively.
555 556 let's say 'C1', 'C2', 'C3' are next mounted at the same dentry 'b' on
557 mount 'B1', 'B2' and 'B3' respectively.
558 559 if 'C1' is unmounted, all the mounts that are most-recently-mounted on
560 'B1' and on the mounts that 'B1' propagates-to are unmounted.
561 562 'B1' propagates to 'B2' and 'B3'. And the most recently mounted mount
563 on 'B2' at dentry 'b' is 'C2', and that of mount 'B3' is 'C3'.
564 565 So all 'C1', 'C2' and 'C3' should be unmounted.
566 567 If any of 'C2' or 'C3' has some child mounts, then that mount is not
568 unmounted, but all other mounts are unmounted. However if 'C1' is told
569 to be unmounted and 'C1' has some sub-mounts, the umount operation is
570 failed entirely.
571 5725g) Clone Namespace
573 574 A cloned namespace contains all the mounts as that of the parent
575 namespace.
576 577 Let's say 'A' and 'B' are the corresponding mounts in the parent and the
578 child namespace.
579 580 If 'A' is shared, then 'B' is also shared and 'A' and 'B' propagate to
581 each other.
582 583 If 'A' is a slave mount of 'Z', then 'B' is also the slave mount of
584 'Z'.
585 586 If 'A' is a private mount, then 'B' is a private mount too.
587 588 If 'A' is unbindable mount, then 'B' is a unbindable mount too.
589 590 5916) Quiz
592 593 A. What is the result of the following command sequence?
594 595 mount --bind /mnt /mnt
596 mount --make-shared /mnt
597 mount --bind /mnt /tmp
598 mount --move /tmp /mnt/1
599 600 what should be the contents of /mnt /mnt/1 /mnt/1/1 should be?
601 Should they all be identical? or should /mnt and /mnt/1 be
602 identical only?
603 604 605 B. What is the result of the following command sequence?
606 607 mount --make-rshared /
608 mkdir -p /v/1
609 mount --rbind / /v/1
610 611 what should be the content of /v/1/v/1 be?
612 613 614 C. What is the result of the following command sequence?
615 616 mount --bind /mnt /mnt
617 mount --make-shared /mnt
618 mkdir -p /mnt/1/2/3 /mnt/1/test
619 mount --bind /mnt/1 /tmp
620 mount --make-slave /mnt
621 mount --make-shared /mnt
622 mount --bind /mnt/1/2 /tmp1
623 mount --make-slave /mnt
624 625 At this point we have the first mount at /tmp and
626 its root dentry is 1. Let's call this mount 'A'
627 And then we have a second mount at /tmp1 with root
628 dentry 2. Let's call this mount 'B'
629 Next we have a third mount at /mnt with root dentry
630 mnt. Let's call this mount 'C'
631 632 'B' is the slave of 'A' and 'C' is a slave of 'B'
633 A -> B -> C
634 635 at this point if we execute the following command
636 637 mount --bind /bin /tmp/test
638 639 The mount is attempted on 'A'
640 641 will the mount propagate to 'B' and 'C' ?
642 643 what would be the contents of
644 /mnt/1/test be?
645 6467) FAQ
647 648 Q1. Why is bind mount needed? How is it different from symbolic links?
649 symbolic links can get stale if the destination mount gets
650 unmounted or moved. Bind mounts continue to exist even if the
651 other mount is unmounted or moved.
652 653 Q2. Why can't the shared subtree be implemented using exportfs?
654 655 exportfs is a heavyweight way of accomplishing part of what
656 shared subtree can do. I cannot imagine a way to implement the
657 semantics of slave mount using exportfs?
658 659 Q3 Why is unbindable mount needed?
660 661 Let's say we want to replicate the mount tree at multiple
662 locations within the same subtree.
663 664 if one rbind mounts a tree within the same subtree 'n' times
665 the number of mounts created is an exponential function of 'n'.
666 Having unbindable mount can help prune the unneeded bind
667 mounts. Here is a example.
668 669 step 1:
670 let's say the root tree has just two directories with
671 one vfsmount.
672 root
673 / \
674 tmp usr
675 676 And we want to replicate the tree at multiple
677 mountpoints under /root/tmp
678 679 step2:
680 mount --make-shared /root
681 682 mkdir -p /tmp/m1
683 684 mount --rbind /root /tmp/m1
685 686 the new tree now looks like this:
687 688 root
689 / \
690 tmp usr
691 /
692 m1
693 / \
694 tmp usr
695 /
696 m1
697 698 it has two vfsmounts
699 700 step3:
701 mkdir -p /tmp/m2
702 mount --rbind /root /tmp/m2
703 704 the new tree now looks like this:
705 706 root
707 / \
708 tmp usr
709 / \
710 m1 m2
711 / \ / \
712 tmp usr tmp usr
713 / \ /
714 m1 m2 m1
715 / \ / \
716 tmp usr tmp usr
717 / / \
718 m1 m1 m2
719 / \
720 tmp usr
721 / \
722 m1 m2
723 724 it has 6 vfsmounts
725 726 step 4:
727 mkdir -p /tmp/m3
728 mount --rbind /root /tmp/m3
729 730 I wont' draw the tree..but it has 24 vfsmounts
731 732 733 at step i the number of vfsmounts is V[i] = i*V[i-1].
734 This is an exponential function. And this tree has way more
735 mounts than what we really needed in the first place.
736 737 One could use a series of umount at each step to prune
738 out the unneeded mounts. But there is a better solution.
739 Unclonable mounts come in handy here.
740 741 step 1:
742 let's say the root tree has just two directories with
743 one vfsmount.
744 root
745 / \
746 tmp usr
747 748 How do we set up the same tree at multiple locations under
749 /root/tmp
750 751 step2:
752 mount --bind /root/tmp /root/tmp
753 754 mount --make-rshared /root
755 mount --make-unbindable /root/tmp
756 757 mkdir -p /tmp/m1
758 759 mount --rbind /root /tmp/m1
760 761 the new tree now looks like this:
762 763 root
764 / \
765 tmp usr
766 /
767 m1
768 / \
769 tmp usr
770 771 step3:
772 mkdir -p /tmp/m2
773 mount --rbind /root /tmp/m2
774 775 the new tree now looks like this:
776 777 root
778 / \
779 tmp usr
780 / \
781 m1 m2
782 / \ / \
783 tmp usr tmp usr
784 785 step4:
786 787 mkdir -p /tmp/m3
788 mount --rbind /root /tmp/m3
789 790 the new tree now looks like this:
791 792 root
793 / \
794 tmp usr
795 / \ \
796 m1 m2 m3
797 / \ / \ / \
798 tmp usr tmp usr tmp usr
799 8008) Implementation
801 8028A) Datastructure
803 804 4 new fields are introduced to struct vfsmount
805 ->mnt_share
806 ->mnt_slave_list
807 ->mnt_slave
808 ->mnt_master
809 810 ->mnt_share links together all the mount to/from which this vfsmount
811 send/receives propagation events.
812 813 ->mnt_slave_list links all the mounts to which this vfsmount propagates
814 to.
815 816 ->mnt_slave links together all the slaves that its master vfsmount
817 propagates to.
818 819 ->mnt_master points to the master vfsmount from which this vfsmount
820 receives propagation.
821 822 ->mnt_flags takes two more flags to indicate the propagation status of
823 the vfsmount. MNT_SHARE indicates that the vfsmount is a shared
824 vfsmount. MNT_UNCLONABLE indicates that the vfsmount cannot be
825 replicated.
826 827 All the shared vfsmounts in a peer group form a cyclic list through
828 ->mnt_share.
829 830 All vfsmounts with the same ->mnt_master form on a cyclic list anchored
831 in ->mnt_master->mnt_slave_list and going through ->mnt_slave.
832 833 ->mnt_master can point to arbitrary (and possibly different) members
834 of master peer group. To find all immediate slaves of a peer group
835 you need to go through _all_ ->mnt_slave_list of its members.
836 Conceptually it's just a single set - distribution among the
837 individual lists does not affect propagation or the way propagation
838 tree is modified by operations.
839 840 All vfsmounts in a peer group have the same ->mnt_master. If it is
841 non-NULL, they form a contiguous (ordered) segment of slave list.
842 843 A example propagation tree looks as shown in the figure below.
844 [ NOTE: Though it looks like a forest, if we consider all the shared
845 mounts as a conceptual entity called 'pnode', it becomes a tree]
846 847 848 A <--> B <--> C <---> D
849 /|\ /| |\
850 / F G J K H I
851 /
852 E<-->K
853 /|\
854 M L N
855 856 In the above figure A,B,C and D all are shared and propagate to each
857 other. 'A' has got 3 slave mounts 'E' 'F' and 'G' 'C' has got 2 slave
858 mounts 'J' and 'K' and 'D' has got two slave mounts 'H' and 'I'.
859 'E' is also shared with 'K' and they propagate to each other. And
860 'K' has 3 slaves 'M', 'L' and 'N'
861 862 A's ->mnt_share links with the ->mnt_share of 'B' 'C' and 'D'
863 864 A's ->mnt_slave_list links with ->mnt_slave of 'E', 'K', 'F' and 'G'
865 866 E's ->mnt_share links with ->mnt_share of K
867 'E', 'K', 'F', 'G' have their ->mnt_master point to struct
868 vfsmount of 'A'
869 'M', 'L', 'N' have their ->mnt_master point to struct vfsmount of 'K'
870 K's ->mnt_slave_list links with ->mnt_slave of 'M', 'L' and 'N'
871 872 C's ->mnt_slave_list links with ->mnt_slave of 'J' and 'K'
873 J and K's ->mnt_master points to struct vfsmount of C
874 and finally D's ->mnt_slave_list links with ->mnt_slave of 'H' and 'I'
875 'H' and 'I' have their ->mnt_master pointing to struct vfsmount of 'D'.
876 877 878 NOTE: The propagation tree is orthogonal to the mount tree.
879 8808B Locking:
881 882 ->mnt_share, ->mnt_slave, ->mnt_slave_list, ->mnt_master are protected
883 by namespace_sem (exclusive for modifications, shared for reading).
884 885 Normally we have ->mnt_flags modifications serialized by vfsmount_lock.
886 There are two exceptions: do_add_mount() and clone_mnt().
887 The former modifies a vfsmount that has not been visible in any shared
888 data structures yet.
889 The latter holds namespace_sem and the only references to vfsmount
890 are in lists that can't be traversed without namespace_sem.
891 8928C Algorithm:
893 894 The crux of the implementation resides in rbind/move operation.
895 896 The overall algorithm breaks the operation into 3 phases: (look at
897 attach_recursive_mnt() and propagate_mnt())
898 899 1. prepare phase.
900 2. commit phases.
901 3. abort phases.
902 903 Prepare phase:
904 905 for each mount in the source tree:
906 a) Create the necessary number of mount trees to
907 be attached to each of the mounts that receive
908 propagation from the destination mount.
909 b) Do not attach any of the trees to its destination.
910 However note down its ->mnt_parent and ->mnt_mountpoint
911 c) Link all the new mounts to form a propagation tree that
912 is identical to the propagation tree of the destination
913 mount.
914 915 If this phase is successful, there should be 'n' new
916 propagation trees; where 'n' is the number of mounts in the
917 source tree. Go to the commit phase
918 919 Also there should be 'm' new mount trees, where 'm' is
920 the number of mounts to which the destination mount
921 propagates to.
922 923 if any memory allocations fail, go to the abort phase.
924 925 Commit phase
926 attach each of the mount trees to their corresponding
927 destination mounts.
928 929 Abort phase
930 delete all the newly created trees.
931 932 NOTE: all the propagation related functionality resides in the file
933 pnode.c
934 935 936------------------------------------------------------------------------
937 938version 0.1 (created the initial document, Ram Pai linuxram@us.ibm.com)
939version 0.2 (Incorporated comments from Al Viro)
940